In a synchronous motor using a permanent magnet, torque is generated by the interaction between the magnetic force of the permanent magnet used in the rotor and the magnetic force generated by passing a current through the windings wound around the stator, and the rotor is thus rotated. The generated torque is generally in proportion to the magnitude of the current. However, when a large current is made to flow in order to obtain larger torque, irreversible demagnetization is caused to the permanent magnet of the rotor by the strong magnetic flux generated by the stator, and thus the magnetic flux generated by the rotor is reduced and the torque is also reduced.
In particular, in a rotor with a permanent magnet that has polar anisotropic orientation, the thickness in the radial direction of the inter-magnetic pole portions is small and the easy-magnetization direction of the inter-magnetic pole portions is in the rotation direction. Therefore, when a current is made to flow through the winding, the magnetic flux concentrates on a slot opening of a stator core and, because of the opposing magnetic field generated by the stator, the permanent magnet of the inter-magnetic pole portion is directly exposed to an opposing magnetic field, which means that irreversible demagnetization can easily occur.
In the conventional synchronous motor described in Patent Literature 1 listed below, grooves extending in an axial direction are formed on the inter-magnetic pole portions on the surface of the outer circumferential surface of the permanent magnet, with the surface being opposed to the stator core. By providing the grooves on the outer circumferential surface of the rotor, the conventional synchronous motor can suppress the reduction in the amount of magnetic flux caused by irreversible demagnetization of the permanent magnet due to the application of an opposing magnetic field.